Implement visitation over the decl context.
Implement visitation over the decl context.

The decl context is the 'container' of the decls either an enum, a class, a struct
an union or a namespace. We can default to it in almost all cases for reverting.

marsupial/cling,root-mirror/cling,perovic/cling,perovic/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling,karies/cling,marsupial/cling,perovic/cling,karies/cling,perovic/cling,karies/cling,marsupial/cling,root-mirror/cling,root-mirror/cling,marsupial/cling,marsupial/cling,karies/cling,marsupial/cling,perovic/cling,karies/cling,karies/cling